Most moderate to high power solid state lasers, such as those used for space-borne laser remote sensing systems, require high-power pulsed or quasi-continuous-wave (“Quasi-CW” or “QCW”) laser diodes as their optical pump source. One or more laser diodes may be used. If more than one laser diode is used, the laser diodes may be structured in a laser diode bar and laser diode bars can be stacked to form a two dimensional array, referred to as Laser Diode Array (LDA). For purposes of this application, the terms laser diode and laser diode array will be used interchangeably. LDA performance and reliability directly determine the effective system operational lifetime, as the laser transmitter operational lifetime is the instrument's lifetime, for without the transmitted beam, there is no reflected signal to record or analyze. For example, statistical analysis of lifecycle testing of an LDA used for an Earth-orbiting two-micron LIDAR (light detection and ranging) instrument indicates that there is a 1% probability that such an LDA will fail before it accumulates 30 million shots, a 50% probability of failing before reaching 120 million shots, and a 99.9% probability of failing before reaching 220 million shots. This lifetime is inadequate for such an Earth-orbiting LIDAR instrument which will require a lifetime of at least one billion shots. As for nearly all electronic devices, the higher the device's temperature, the shorter the lifetime.
In order to minimize the risk to such missions and other semiconductor laser based instruments, there is a need to better understand and design a means to reduce the probability of failure of the LDAs. The production of the useful radiation is not 100% efficient (typically 50%) and the laser diodes produce heat, which causes an overall rise in their temperature and requires some means to keep the diodes from overheating which could result in catastrophic failure. Compared with their low-power CW counterparts, these LDAs suffer from shorter lifetimes and are more susceptible to degradation and premature failure. The primary factor in their rapid degradation and failure is the excessive localized heating and substantial pulse-to-pulse thermal cycling of the laser active regions when such devices are operated at high currents over a relatively long pulse duration (beyond 0.2 milliseconds), even at relatively low pulse repetition frequencies. For example, the thermally-induced stresses are particularly significant when the required pump pulsewidth is increased from 200 microseconds (required for neodymium-based lasers) to at least one millisecond (required for thulium and holmium lasers). If the laser diode's junction temperature can be monitored during operation (i.e., while the laser is being operated, but during the intervals between drive pulses and not while the drive pulses are occurring), this information would be useful in determining overall laser diode health, expected lifetime, and problems with the instrument's cooling system. Detection of excessive temperature rise during the instrument operation can also allow for preventive measures to prolong the LDA lifetime, such as reducing drive current or pulse duration and bypassing defective laser diode bars or arrays. Additionally, this invention provides a reliable and accurate means for screening and evaluating LDAs prior to utilization in an instrument.
Until now there have been only indirect or relative means of measuring the junction temperature during a pulse, such as measuring the change in the optical power, peak wavelength or a shift in the threshold current necessary to cause the medium to lase. Other means are either too slow (such as infrared photography) or too large (such as thermocouples or thermistors) to accurately and effectively measure the temperature of a junction (which is only several microns thick). None of these techniques are practical for real time monitoring of the LDA junction temperature during the instrument operation as they either intrude into the optical path or require extensive data processing.
The ability to measure the junction temperature of high power LDAs is crucial to determining the reliability and lifetime of these devices and monitoring their operation. Junction temperature measurement is also vital to being able to quantify any improvements that are made in manufacturing processes, device materials, and laser architecture (especially that of the laser cooling systems).